1. Field of the Invention
The present invention relates generally to the field of electronic (radar) missile guidance. More particularly, the present invention relates to a semi-active, high duty cycle, pulsed, pseudonoise system for missile guidance.
2. Description of the Prior Art
One of the major problems associated with the operation of a semiactive continuous wave guidance system is the illuminator leakage which competes with the target return echo in the missile receiving system. The said leakage signal level may be 100 decibels or more above the desired echo power level, and is of variable strength as a function of the relative ranges between missile and the ground illuminator on the one hand, and the missile and the target on the other hand. The actual power ratio between this undesired leakage and the illuminated target re-radiation which is to be detected also depends on whether or not the missile is in the illuminator antenna main beam or one of its side lobes. The large leakage signal can and does cause saturation in the receiver which can have the affect of "blinding" the missile.
Of course, the echo from the target, as received at the missile front receiver, will exhibit a doppler frequency shift in excess of that received by the missile as leakage from the ground illuminator. This is due to the fact that the leakage doppler results only from the missile's own velocity, whereas the front receiver signal exhibits a doppler shift as a function of the closing velocity between the missile and target. That fact can be, and has been, used to separate the two signals; however, leakage noise sidebands can readily fall into the system echo doppler band with the result that missile performance tends to deteriorate no matter how sharp the frequency discriminating circuits might be.
It has been demonstrated that the affect of the aforementioned leakage can be reduced by utilizing pseudonoise (PN) modulation on the CW carrier. That technique is described in U.S. Pat. No. 3,641,573. However, due to other system constraints, such as lock on time, when using that type of modulation, the realized improvement (on the order of 20 or 30 decibels) may not always be sufficient.
It should be noted that the reason for utilizing pseudonoise modulation in a CW semi-active system of the type to which the present invention relates is not primarily leakage reduction. Rather, the pseudonoise modulation technique in such CW semi-active systems has been recognized as a vehicle for improving performance in certain electronic environments. Such improvements are due to the autocorrelation function of the PN modulation which affords the system range, as well as doppler, discrimination.
Accordingly, it will be recognized as highly desirable to retain the PN modulation while totally eliminating the aforementioned leakage at least insofar as its affect on the system is concerned.